A polyester film, in particular a biaxially stretched film of polyethylene terephthalate, polyethylene naphthalate or the like, is widely used as a material of a magnetic tape, a ferromagnetic thin film tape, a photographic film, a wrapping film, an electronics part film, an electrically insulating film, a film for lamination with a metal, a glass display and the like, since it is excellent in mechanical property, heat resistance and chemical resistance.
In an image display device referred to as a flat panel display, which is represented by a liquid crystal display, a glass substrate is used as a supporting substrate for various kinds of functional layers, such as a display element and a transparent electrode. In recent years, however, an image display device is demanded to have a low-profile, a lightweight, a large screen size, a flexibility in shape, a curved display and the like, and therefore, it is being considered to use a highly transparent polymer film substrate having a light weight and flexibility instead of a glass substrate, which has a heavy weight and is liable to be broken.
In an image display device, a liquid crystal display requires a large number of parts since a backlight is necessarily employed, but a self-emitting device, which is represented by an organic EL, is being positively developed since it can be fabricated with a small number of parts. Such a demand is increased in this purpose that the breakable property and the heavy weight, as the defects of glass, are eliminated to attain flexibility, and thus a polymer film is being employed therein.
The polymer film has, depending on purposes, various functional layers, such as a gas barrier layer, a conductor layer, a semiconductor layer and a luminescent layer, laminated. The functional layers are laminated by such a method as vapor deposition, ion plating, sputtering and plasma CVD. While the temperature varies depending on the lamination method, the polymer film is exposed to a significantly high temperature, and therefore, the polymer film suffers, in some cases, thermal expansion due to temperature change or thermal shrinkage due to a heat treatment at a high temperature. In particular, the dimensional stability to temperature change is one of important characteristics also in a glass substrate, and a low coefficient of linear thermal expansion is being demanded. However, the rate of dimensional change of a polymer film is larger than glass, and is liable to be different from the dimensional change rates of the functional layers. As a result, upon laminating the functional layer to the film or after laminating the functional layer thereto, there arises such a problem that the laminated product is broken due to cracks or wrinkles occurring therein, whereby the functional layers fail to exert the intended functions.
A flexible printed circuit is an electric circuit disposed on a substrate having flexibility, and is formed by adhering a metallic foil to a film as a substrate, or applying plating or the like to the film, and then subjecting the substrate to etching. The substrate having a circuit formed thereon is further subjected to a heat treatment, mounting of circuit parts, and the like, so as to be used practically as an electric or electronics apparatus. In recent years, a portable electric or electronics apparatus, such as a notebook personal computer and a mobile phone, is being spread rapidly, and miniaturization of the apparatus is also being practiced. However, the apparatus having been miniaturized is still demanded to have such functions that are equivalent to or higher than those of the conventional ones, and accordingly, the circuit is demanded to have a small size and a high density. Furthermore, while a demand of price reduction is being increased, a polyimide film, which has been used as a film for a flexible circuit substrate, is expensive and suffers a large dimensional change upon absorbing moisture, and thus, a polymer film as a substitute of a polyamide film is being investigated. A polymer film for a flexible printed circuit substrate is also demanded to have dimensional stability at a high temperature and a small dimensional distortion caused by coefficients of linear thermal expansion between a metallic foil and the substrate film, as similar to the substrate for an image display device.
As for the dimensional stability of a polyester film at a high temperature, JP-A-2004-009362 discloses a polyester film that is controlled in thermal shrinkage rate to have transparency. According to the method, however, the coefficient of linear thermal expansion in a working temperature range is still large although it is excellent in dimensional stability at a high temperature.
JP-A-2002-018947 discloses that a biaxially stretched film having a coefficient of linear thermal expansion and a humidity expansion coefficient in particular ranges is subjected to a thermal relaxing treatment once or more to provide a biaxially stretched polyester film for a photographic light-sensitive material excellent in thermal shrinkage rate. However, JP-A-2002-018947 discloses the thermal relaxing treatment at a high temperature for decreasing the thermal shrinkage rate at 120° C., which is required in a photographic light-sensitive material, and the coefficient of linear thermal expansion is rather deteriorated although the thermal shrinkage rate is improved by the method.
As a result of reduction in thickness of a magnetic recording medium, such as a magnetic tape, associated with miniaturization and prolongation of recording time, shrinkage in the width direction is liable to occur with elongation deformation in the longitudinal direction, which brings about, for example, deviation in recording tracks. In order to avoid the problem, JP-A-2002-205332 discloses such a polyester film that has a coefficient of linear thermal expansion in the film width direction at room temperature to 50° C. in a range of from 0 to 12 ppm/° C. and a thermal shrinkage rate in the film width direction at 100° C. in a range of from 0 to 0.3%, and also discloses a preferred film thickness in a range of from 3 to 7 μm. However, JP-A-2002-205332 intends to control the coefficient of linear thermal expansion in a relatively gentle temperature range of from room temperature to 50° C., and notes that the coefficient of linear thermal expansion in the film longitudinal direction is preferably a negative coefficient of linear thermal expansion, i.e., is preferably in direction of shrinkage. JP-A-11-279293 discloses such a film for a magnetic recording medium as a major purpose that has coefficients of linear thermal expansion and humidity expansion coefficients in the film longitudinal and width directions and a thermal shrinkage rate at 65° C. in prescribed ranges and has a film thickness of 7 λm or less. However, JP-A-11-279293 only discloses preferred relationship among the characteristics but does not disclose specific ranges for the coefficients of linear thermal expansion, and only a negative coefficient of linear thermal expansion in the longitudinal direction is disclosed in the example.
As having been described, it is the current situation that it is difficult to obtain such a film that is excellent in dimensional stability of both a thermal shrinkage rate and a coefficient of linear thermal expansion in the film longitudinal and width directions.